Density functional theory and electron paramagnetic resonance study on the effect of N-F codoping of TiO2

Recent experiments have indicated that titanium dioxide (TiO2) codoped with nitrogen and fluorine may show enhanced photocatalytic activity in the visible region with respect to TiO2 doped only with nitrogen. Prompted by these findings, we have investigated N-F codoped TiO2 through a combined theoretical and experimental study. Density functional theory (DFT) calculations have been carried out both within the generalized gradient approximation (GGA) and using hybrid functionals to accurately describe the electronic structure; substitutional as well as interstitial locations of nitrogen in the TiO2 lattice were considered. From these calculations we infer that N-F codoping reduces the. energy cost of doping and also the amount of defects (number of oxygen vacancies) in the lattice, as a consequence of the charge compensation between the nitrogen (p-dopant) and the fluorine (n-dopant) impurities. The UV-visible spectra of the sol-gel prepared TiO2 powders confirm the synergistic effect of N-F codoping: more impurities are introduced in the lattice with an increased optical absorption in the visible. EPR spectroscopy measurements on the codoped samples identify two paramagnetic species which are associated to bulk N impurities (N-b(center dot)) and Ti3+ ions. Preliminary photocatalytic tests also indicate an enhanced activity under vis-light irradiation toward degradation of methylene blue for the codoped system with respect to N-doped TiO2.